An organic compound, compared with an inorganic compound, is rich in variety of material system and has a possibility to synthesize materials having various functions by a suitable molecular design. In addition, it has characteristics such that formed things thereof such as a film are rich in flexibility and, further, processability is good by forming a high molecular weight. Due to these advantages, recently, photonics and electronics employing functional organic material have grabbed attention.
For example, a solar cell and a light emitting element (also referred to as an organic electroluminescent device) can be mentioned as examples of a photoelectronics device employing an organic semiconductor material as a functional organic material. These are devices taking advantage of electric properties (carrier transport properties) and optical properties (light absorption or light emission) of the organic semiconductor material. Among them, in particular, a light emitting element has made remarkable development.
The light emitting element is constituted by sandwiching a layer containing a light emitting material between a pair of electrodes (anode and cathode). The light emitting mechanism thereof is said that a hole injected from an anode and an electron injected from a cathode when a voltage is applied between both electrodes recombine in a layer containing a light emitting material to recombine at an emission center in the layer containing the light emitting material to form a molecular exciton, and that the molecular exition emits energy upon returning to the ground state to emit light. As for the exited state, the singlet excitation and triplet excitation are known, and light emission is thought to be possible via either excited state.
The layer containing a light emitting material may be a single layer structure of only a light emitting layer composed of a light emittable material, or may be formed by laminating a hole injecting layer, a hole transporting layer, a hole blocking layer, an electron transporting layer, an electron injecting layer and the like composed of plural functional materials, in addition to the light emitting layer. In the light emitting layer, it is possible to change arbitrarily hue of the emitted light by doping a guest material into a host material. In addition, some combinations of a host material and a guest material have the potential to improve brightness and lifetime of the light emitting.
As for the material for use in a layer containing the light emitting material, many materials having various structures and functions are employed. Among them, CBP (4,4-di(N-carbazole)biphenyl), PVK (polyvinyl carbazole) and the like are known as a material (carbazole derivative) having a carbazole skeleton with an excellent photoconductivity, and are used aboundingly still at the present day.
CBP, which is a low molecular weight material, is formed into a film mainly by an evaporation method and is aboundingly employed as a host material having hole transporting properties in the light emitting layer (for example, refer to Patent Document 1).
(Patent Document 1) JP-A-2001-244077
However, although the material has the characteristic of a high thermal property value (good heat resistance), it has a demerit such that maintenance of an amorphous state is difficult upon forming a film to crystallize easily.
On the other hand, PVK, which is a high molecular weight material, is formed into a film mainly by a wet process such as a coating method (including spin coating method) and a ink jet method, and are employed aboundingly as a host material in the light emitting layer similar to CBP (for example, refer to Patent Document 2).
(Patent Document 2) JP-A-2001-257076
When used as a host material in the light emitting layer, a high molecular weight material has the characteristic such that it excels in brightness properties (maximum brightness is several tens of thousands cd/m2) compared with a low molecular weight material. However, it has demerits such that it has a bad heat resistance and low reliability, as well as can be formed into a film only by limited methods.
Accordingly, both of the low molecular weight material and the high molecular weight material having a carbazole skeleton have aforementioned demerits respectively, and development of a material overcoming these demerits is desired in order to improve further element properties of the light emitting element.
Thus, the present invention aims to provide a carbazole derivative which has an excellent heat resistance and is difficult to crystallize when formed into a film. In addition, the invention aims to accomplish extending lifetime of an organic semiconductor element and a light emitting element by producing the organic semiconductor element and the light emitting element while employing the carbazole derivative.